In a continuously variable transmission (CVT), the variation of speed ratio when an accelerator pedal is fully depressed is set for example as follows.
First, the speed ratio is changed to a maximum value, i.e., the speed ratio at which an output rotation speed is lowest relative to an input rotation speed. Second, the engine rotation speed is increased to a maximum region, and a running speed is then increased by decreasing the speed ratio.
In this speed ratio control, the engine rotation speed remains in the maximum region for a long period from when the accelerator pedal is depressed to when the vehicle speed reaches the speed desired by the driver, and this causes a large noise outside the vehicle.
In order to solve this noise problem, Tokkai Hei 5-126239 published by the Japanese Patent office in 1993 discloses a method wherein a transient target speed ratio is set so that the engine rotation speed increases gradually for a sudden operation of the accelerator pedal.
According to this method, when the accelerator pedal is depressed, the engine rotation speed increases gradually due to the fact that the speed ratio changes to the maximum value comparatively slowly. In the period required to reach the vehicle speed depending on depression of the accelerator pedal, the time during which the engine rotation speed is in the maximum region is short, so vehicle noise due to acceleration is less.